Urokinase


Urokinase, also known as urokinase-type plasminogen activator, is a serine protease present in humans and other animals. The human urokinase protein was discovered, but not named, by McFarlane and Pilling in 1947. Urokinase was originally isolated from human urine, and it is also present in the blood and in the extracellular matrix of many tissues. The primary physiological substrate of this enzyme is plasminogen, which is an inactive form of the serine protease plasmin. Activation of plasmin triggers a proteolytic cascade that, depending on the physiological environment, participates in thrombolysis or extracellular matrix degradation. This cascade had been involved in vascular diseases and cancer progression.
Urokinase is encoded in humans by the PLAU gene, which stands for "plasminogen activator, urokinase". The same symbol represents the gene in other animal species.

Function

The PLAU gene encodes a serine protease involved in degradation of the extracellular matrix and possibly tumor cell migration and proliferation. A specific polymorphism in this gene may be associated with late-onset Alzheimer disease and also with decreased affinity for fibrin-binding. The protein encoded by this gene converts plasminogen to plasmin by specific cleavage of an Arg-Val bond in plasminogen. This gene's proprotein is cleaved at a Lys-Ile bond by plasmin to form a two-chain derivative in which a single disulfide bond connects the amino-terminal A-chain to the catalytically active, carboxy-terminal B-chain. This two-chain derivative is also called HMW-uPA. HMW-uPA can be further processed into LMW-uPA by cleavage of chain A into a short chain A and an amino-terminal fragment. LMW-uPA is proteolytically active but does not bind to the uPA receptor.

Structure

Urokinase is a 411-residue protein, consisting of three domains: the serine protease domain, the kringle domain, and the growth factor domain. Urokinase is synthesized as a zymogen form, and is activated by proteolytic cleavage between Lys158 and Ile159. The two resulting chains are kept together by a disulfide bond.

Interaction partners

The most important inhibitors of urokinase are the serpins plasminogen activator inhibitor-1 and plasminogen activator inhibitor-2, which inhibit the protease activity irreversibly. In the extracellular matrix, urokinase is tethered to the cell membrane by its interaction to the urokinase receptor.
uPa also interacts with protein C inhibitor.

Urokinase and cancer

Elevated expression levels of urokinase and several other components of the plasminogen activation system are found to be correlated with tumor malignancy. It is believed that the tissue degradation following plasminogen activation facilitates tissue invasion and, thus, contributes to metastasis. Urokinase-type plasminogen activator is more commonly associated with cancer progression than tissue plasminogen activator. This makes uPA an attractive drug target, and, so, inhibitors have been sought to be used as anticancer agents. However, incompatibilities between the human and murine systems hamper clinical evaluation of these agents. Moreover, urokinase is used by normal cells for tissue remodeling and vessel growth, which necessitates distinguishing cancer-associated urokinase features for specific targeting.
uPA breakdown of the extracellular matrix is crucial for initiating the angiogenesis which is associated with cancer growth.
uPA antigen is elevated in breast cancer tissue, which correlates with poor prognosis in breast cancer patients. For this reason, uPA can be used as a diagnostic biomarker in breast cancer.
Through its interaction with the urokinase receptor, urokinase affects several other aspects of cancer biology such as cell adhesion, migration, and cellular mitotic pathways.
As of December 7, 2012, Mesupron, a small molecule serine protease inhibitor developed by the WILEX pharmaceutical company, has completed phase II trials. Mesupron appears to be safe when combined with chemotherapeutic drug Capecitabine for the progression-free survival in human breast cancer.

Clinical applications

Urokinase is effective for the restoration of flow to intravenous catheters blocked by clotted blood or fibrin. Catheters are used extensively to administer treatments to patients for such purposes as dialysis, nutrition, antibiotic treatment and cancer treatment. Approximately 25% of catheters become blocked, meaning that affected patients cannot receive treatment until the catheter has been cleared or replaced. Urokinase is also used clinically as a thrombolytic agent in the treatment of severe or massive deep venous thrombosis, peripheral arterial occlusive disease, pulmonary embolism, acute myocardial infarction, and occluded dialysis cannulas. It is also administered intrapleurally to improve the drainage of complicated pleural effusions and empyemas. Urokinase is marketed as Kinlytic and competes with recombinant tissue plasminogen activator as a thrombolytic drug.
All plasminogen activators catalyze the production of plasmin, which in turn leads to the breakdown of the fibrin lattice structure in blood clots.  While there are commonalities in the mode of action for urokinase and tPA, urokinase has some advantages for treatment of peripheral clots.
Unlike tPA, which is activated by binding to the fibrin within clots, urokinase is not sequestered by fibrin and therefore does not specifically attack hemostatic clots.  This makes urokinase less likely to break down such hemostatic clots that are essential for ongoing blood vessel repair throughout the body.  Dissolution of these “good” clots can lead to serious adverse events through hemorrhagic bleeding.  Years of clinical study have confirmed the safety advantage of using urokinase. Consequently, urokinase has been preferentially used in deep venous thrombosis and peripheral arterial occlusive disease where it is administered directly to the site of the clot while tPA is preferred in AMI where peripheral bleeding is a secondary consideration.

Society and culture

The presence of a fibrinolytic enzyme in human urine was reported in 1947, without a name given for such an enzyme behind its effect. In 1952 a purified form of the enzyme was extracted from human urine and named "urokinase" for "urinary kinase". The full text for this article is lost, and the only citation points to the abstract of a list of papers read at a conference in the same journal. A few other papers on the purification were published independently around the same time. By 1960, it was still unclear whether the activation of plasminogen has anything to do with a protease, but a kinase is thought to play a role regardless.